A rotameter is a variable area flow meter used to measure and control the flow of liquid through a conduit. Rotameters include a flow tube that has a tapered, e.g., conical, configuration. The rotameter is mounted vertically with the narrow end of the tapered flow tube at the bottom and the widened end at the top. The rotameter is arranged in the fluid circuit such that fluid flow passes through the flow tube from bottom to top.
Rotameters include a non-buoyant indicator, such as a piston or pellet, that is positioned in the flow tube and movable in the tube along an axis that coincides with the direction of flow through the tube. The indicator can be simply disposed in the flow tube and movable freely therein. This can be the case, for example, with smaller, relatively low flow rate rotameters in which the indicator is a pellet-type indicator. The indicator can also be supported in the flow tube, for example, by a rod that is centrally disposed in the flow tube and along which the indicator can slide freely. This can be the case, for example, with a larger, relatively high flow rate rotameters in which the indicator is a piston-type indicator. Regardless of the configuration, the indicator is configured so that there is a clearance, typically an annular clearance, between the flow tube and the indicator.
In use, fluid flows into the rotameter through an inlet, enters the flow tube at or near the bottom, travels upward along the tube, and exits the tube through an outlet at or near the top of the rotameter. As the fluid flows upward along the flow tube, it flows around the indicator while, at the same time, acts on the indicator and pushes or otherwise urges the indicator to move upward in the flow tube. Because the flow tube is tapered and positioned so that it widens as fluid flows upward in the tube, for a given fluid flow rate, the fluid velocity in the flow tube decreases as it travels up the tube. Thus, for a given flow rate, the magnitude of the force acting on the indicator due to flow within the tube decreases as the indicator travels upward within the tube. Because of this, for a given flow rate, there is a point within the flow tube where the weight of the indicator is in balance with the force exerted by the flow.
Due to these principles of operation, the position of the indicator in the flow tube corresponds to the rate of fluid flow through the rotameter. The correlation between indicator position and flow through the rotameter can be indicated by viewing the position of the indicator against indicia on the rotameter body indicative of the flow rate associated with that position. This indicia can, for example, be a graduated scale adjacent a translucent window through which the indicator can be viewed.
By constructing the rotameter body, i.e., the flow tube, with precise dimensions, and constructing the indicator with a consistent shape and weight, the correlation between indicator position and flow through the meter can be calculated and the indicia applied accordingly. These can be verified by testing, for example, in a metrology lab. In many constructions, the rotameter body itself is constructed of a cast translucent acrylic material, which provides high strength for withstanding fluid pressures, allows for casting the tapered flow tube with precise dimensions, and permits the indictor to be viewed directly through the rotameter body.
A rotameter in combination with a valve can be used to meter or otherwise control fluid flow through a conduit. Adjusting the valve while viewing the rotameter allows a user to select a desired flow through the conduit. A valve capable of fine adjustments can produce correspondingly fine adjustments to the flow rate through the rotameter. In some constructions, the valve can be an integrated component of the rotameter design. In this case, the rotameter can be used to control and monitor fluid flow with a high degree of precision.
As rotameters increase in size to accommodate higher flow rates, their cost increases exponentially due to the cost of the acrylic material required to cast the body of the meter. The increased size of the valve required for these larger rotameter constructions can increase the amount of acrylic material necessary for its construction. Since conventional rotameter constructions adopt an elongated rectangular configuration, an increase in valve size can require an increase in cross-sectional area of the rotameter body not only in the area of the valve, but along its entire length. Therefore, it can be desirable to construct an integrated rotameter valve that is compact in size and allows for fine adjustments.